Optical systems, such as imaging systems, telecommunications devices, micro-optical systems, micro-mechanical systems, etc., are typically constructed of several different lenses and optical articles to deliver the desired optical performance. To avoid large overall losses in the optical system, the alignment of each lens and optical article with subsequent lenses and optical articles must be very precise. Fiducial marks are often created on the lenses and optical articles outside the optical area to serve as a reference point during alignment. Fiducial marks are particularly important in the case of aspheric lenses and lens arrays where it is difficult to identify the center of the lens during alignment activities. Fiducial marks are also very important for fiber optic arrays and laser arrays where multiple features dictate the need for a shared alignment reference which is located precisely in relation to all the optical features. As optical systems get smaller for fiber optics applications, like telecommunications and optical sensors, the need increases for precise alignment of the optical components and the accuracy of the associated fiducial marks. Alignment specifications of two (2) microns are now common with a desire to deliver submicron alignment accuracy. Consequently, the fiducial marks must be located with an accuracy of 1 micron or better.
Fiducial marks are well known in the semi-conductor manufacturing industry as an important tool for making multilayer semiconductors. In this case, the fiducial marks are incorporated as part of the semiconductor circuit plan. Due to the thinness (50-100 micron) of the semiconductor layers used in making multilayer semiconductors, the fiducial marks of multiple semiconductor layers can be viewed simultaneously using a high magnification microscope. The high magnification microscope aids in positioning the fiducial marks of one semiconductor layer over the fiducial marks of another semiconductor layer during the alignment process.
Forming fiducial marks in optical articles raises special challenges in that optical surfaces are typically relatively thick, often well over a 1000 micron in thickness. This is the case even in a microlens array that has microlenses that are well under a millimeter in diameter. The thickness of the microlens array makes it virtually impossible to accurately locate a fiducial mark by looking through the microlens array due to optical limitations. On the one hand, the location accuracy of the fiducial mark relative to the optical article is limited because the fiducial mark is displaced by refracted light passing through the microlens array material. Moreover, the thickness of the microlens array limits how close the microscope used for identifying the microlens array can be positioned to the fiducial mark. Consequently, only lower magnification microscopes can be used to look at the fiducial. Therefore, for optical articles, a method of applying a very accurately located fiducial mark on the side opposite to the optical article is needed.
In U.S. Pat. No. 6,005,294, by Tsuji et al., Dec. 21, 1999, entitled “Method Of Arranging Alignment Marks,” a method of making semiconductor devices uses multiple fiducial marks in such a way that the area occupied by the fiducial marks is reduced and the manufacturing productivity is correspondingly increased. While this patent does describe the state of the art for making semiconductor devices, the alignment process described therein is not appropriate for optical articles like lens arrays. As mentioned, in lens arrays, the significant thickness of the various lenses makes it impossible to view fiducial marks from multiple optical articles simultaneously due to the separation distance imparted by the material thickness of the lenses.
Also, U.S. Pat. No. 5,850,276, by Ochi et al., Dec. 15, 1998, entitled “Method Of Making LCD Device Having Alignment Mark Made Of Same Material And Formed At Same Time As Microlenses” and U.S. Pat. No. 5,771,085, by Ochi et al., Jun. 23, 1998, entitled “LCD Device With an Alignment Mark Having Same Material As Microlenses” each describe a process for molding fiducial marks into a microlens screen used for liquid crystal display devices. In these patents the shapes of the fiducial marks are also described in detail. The fiducial marks as described are protrusions in the shape of a cross or several other variations, located on the same side as the microlenses. The protrusions can be semicircular in cross section or another shape as long as the grooves between the protrusions stand out as dark lines when viewed with a reflecting microscope. The references recognize that lens characteristics, such as thickness, interfere with the ability to identify underlying fiducial marks. Further, the references show some appreciation for useful geometries of fiducial marks and for fiducial marks molded along with a microlens array. However, neither of the patents show appreciation for fiducial marks applied on the side opposite the optical surfaces in the microlens array. Furthermore, there is no appreciation by either of the references that advantages can be gained with a molded fiducial mark having lens characteristics.
Moreover, U.S. Pat. No. 6,096,155, by Harden et al., Aug. 1, 2000, entitled “Method Of Dicing Wafer Level Integrated Multiple Optical Elements” discloses the use of fiducials to aid in alignment of microlenses on wafers during the bonding of multiple wafers together prior to dicing. This patent generally teaches making integrated multiple optical elements with features to help control the thickness of adhesives and solders used to bond together the wafers. While effective use of the fiducial marks is described, there is absolutely no mention of ways to improve alignment of fiducial marks on one side with the optical element on the other side of the wafer. The techniques of embossing and molding fiducial marks, described in the patent, both suffer from locational inaccuracies from one side to the other o the order of plus or minus ten (10) microns. In molded microlenses and microlens arrays this inaccuracy is not acceptable.
Furthermore, U.S. Pat. No. 4,598,039, by Fischer et al., Jul. 1, 1986, entitled “Formation Of Features In Optical Material” describes the use of a laser to remove optical material in a controlled fashion. The laser can be used directly on the optical material or a layer of ablative absorber material can be put onto the surface of the optical material to enhance the coupling to the laser. This ablative technique is well suited to making fiducial type marks for alignment. However, the reference does not show appreciation for how to align the laser with a lens array that is located on the opposite side from the desired location for the fiducial marks.
Therefore, a need persists in the art for a method of forming fiducial marks onto optical articles and optical arrays on a surface opposite the optical article surface that enables precise alignment of the articles and optical arrays. Moreover, there is a compelling need for a special optical feature molded along with optical surfaces to focus light onto an opposing surface of the optical article or optical array thus enabling the formation of a fiducial mark onto the opposing surface with great accuracy.